The two answers so far have differing advice. Could Cécile comment please?
Laura Lawless’s explanation seems to accord with Alan’s - se parler is one of "20 verbs for which the reflexive pronoun is always an indirect object" (That is, they’re talking to each other, not talking each other, and parlé is invariable.)
https://www.lawlessfrench.com/grammar/agreement-with-pronominal-verbs/
Sorry about the delay in answering you Anne, but I have been researching the answer on 'se parler' which I clearly got wrong.
I have now rectified the previous answer.
https://kwiziq.learnfrenchwithalexa.com/questions/view/ils-se-sont-parle-hier-soir-au-telephone
This topic is extremely complex and I might suggest we cover it in a separate lesson on pronominal verbs.
Let me know if I can help any further.
Bonne Continuation !
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