Both are gramatically correct. If you give me more contexts (e.g. Is this a conversation where you have to call your friend who lives far away from your location?) , I can tell you if you are saying it correctly. Right now your sentences are open to many interpretations.

It depends on a situation. 友達と電話しました ＊「と」is 「with」 友達に電話しました ＊「に」is「to」 それから、「ありがとうございます。」ではなく、「よろしくおねがいします。」です。 参考になれば幸いです。 コミュニティー・チューターJJ

友達「と」電話する／talk WITH my friend on the phone、友達「に」電話する／ make a phone call TO my friend

"電話する" USUALLY means "make a phone call", though it does SOMETIMES mean "talk on the phone" depending on the context. In the latter case, you CAN say "友達と電話する", "彼氏と電話する", and so on. Both of your sentences ARE correct, I'd say.

に電話する means make a phone call to someone from you. When we use 友達と, we say like 友達と電話で話した。友達と電話していた。 These sentences don't show from whom the phone call was made.