In the US, lawyers are ethically prohibited from contacting a party the lawyer knows is represented by counsel regarding a matter that is the subject of that representation. There’s a spirited and enlightening Redline query discussion underway about the practical application of this rule in the transactional context (one in which member jayparkhill commented: “This is super-interesting. It's always good to pull up the rules every once in a while rather that relying on what we think they say.”)
The rule is not without its detractors. Professor Leubsdorf insightfully questions why the consent of the represented party is insufficient to waive operation of the rule. The rule anoints the lawyer as the absolute arbiter of whether the client may contact the other side's lawyer, setting up an inherent conflict of interest:
In US: must be an attorney licensed and in good standing in any state, territory or DC.
Outside US: must be a lawyer or equivalent (eg counselor, barrister, advocate, solicitor), duly educated and licensed/accredited and in good standing.
As a general rule, experienced and currently practicing lawyers, and those in the legal academy, are more likely to be admitted.