There are protocol things that can be done as well:
S(a): I have something for you.
S(b): Not now. Try in x Minutes.
Or
S(a): I have something for you.
S(b): Not now.
S(a) waits a term of their own choosing.
S(a): I have something for you.
S(b): Not now.
S(a): Let me know when you have a sec.
S(b) waiting until load subsides.
S(b): I am ready for you
S(a): I have something for you.
S(b): Thanks.