If you have customers bitching after and install then you didn't set their expectations properly before you sold the system.
SLA is not applicable in an IP PBX environment when it is very easy to have dozens of virtual lines. The global parking system in 3CX version 9 provides the primary function that SLA-type systems had (Joe, pick up call on line 1). Setting that expectation in advance goes a long way towards keeping a customer happy.