The Chesapeake & Potomac Telephone Company has put an end to a time-honored way of saving money on long distance calls, and a number of customers are angry that they weren't told about it.

No longer can a customer place a call during the week at 7:59 a.m., talk for half an hour and have the entire call billed at discount rates. Since last month, one minute of the call is billed at the discount rate and the remaining 29 minutes at peak-hour rates.

There is a good side to the change as well. If a customer makes the call at 4:59 p.m., during the peak period, only one minute will be billed at the peak rate. The rest will be at the lower rate.

What has upset many C & P customers, however, is that they didn't know the company billing policy had changed until they got their bills for April and found that the change had increased their charges for toll calls.

"We are having a moderate number of calls come in from customers complaining that they didn't know about the change," said Web Chamberlin, a C & P representative.

One customer caught in the rule change was Alfred Rich, 66, of Silver Spring. Rich has a longtime habit of placing long distance calls to friends and family before 8 a.m., when toll rates are discounted 60 percent from the regular daytime rates. Sometimes the conversation goes on beyond 8 a.m., which didn't matter before the rule change, but does now.

On April 21, for example, Rich telephoned a friend who lives in a small Florida community called Oklawaha. The call went through at 7:59 a.m. and continued for 14 minutes, according to the telephone bill. Under the old rate plan in which C & P's 60 percent discount applied to all 14 minutes of the conversation, the call would have cost Rich $2.42.

But under the new rate, there is no discount on the 13 minutes of telephone time used after 8 a.m. The discontinuation of that discount--and the nickel increase in the basic toll rate for the call to Oklawaha--resulted in a toll charge for $5.83, twice as much as it was before.

In all, Rich had to pay $49.06 for his April long distance calls--$9.72 more than he would have paid under the old rate structure.

The rate revisions, which took effect April 2, cover a wide range of telephone services and procedures, from increases in basic toll charges to changes in discount prices. Rates have risen for most out-of-state calls, with increases of up to 12 cents for the first minute of time during the full rate period. At the same time, the discount on evening calls from 5 p.m. to 11 p.m. Sunday through Friday has increased from 35 percent to 40 percent.

The cheapest times to make calls are still all day on Saturday, up until 5 p.m. on Sunday, and between 11 p.m. and 8 a.m. during the week. The discount for those times remains 60 percent.

Rich said he wasn't surprised by the rate increase. "But they C & P just didn't let us know that they had changed the timing for the discounts; they made no attempt to educate us so that we then could have taken steps to keep our costs down," he said.

Rich said C & P typically includes in bills a newsletter explaining new technology and other company achievements. "But there was nothing about this change in the newsletter I received," he said.

That is because C & P didn't begin sending notices of the change to Maryland customers until April 19, exactly 17 days after the changes took effect. The notices have been included in bills sent to Virginia and District of Columbia customers since April 20. However, some customers who receive their statements at the end of the C & P billing cycle won't receive their notices until late May.

That will be nearly two months after the April 2 effective date of the rate revisions, which were implemented by the American Telephone & Telegraph Co. under an order approved by the Federal Communications Commission. The rate restructuring automatically extended to the C & P Telephone Co., which is a wholly owned subsidiary of AT&T.

"I would have to say there was an administrative delay in getting those notices out," said Chamberlin, the C & P spokesman. However, he said the company printed notices for customers as soon as possible after the order was approved by the FCC.

The delay, Chamberlin said, was "certainly not deliberate."