Dynamics of Phoneme Addition

Hello everyone,

I'm wondering if there are any rules or at least trends regarding
the dynamics by which one language would borrow phonemes from a
second language with which it is in contact - not just in loanwords
but even in native words.  Can it happen willy-nilly or are there
general rules involved?

Any insight welcome!

Cheers,
Eamon
____________________________________________________
Robert Eamon Graham              robertg@knology.net

Anugraha banana shundarata dengan bisri bastu-bastu.

                                -- U2, "Grace"