In the context of multi-channel call centers with inbound calls and emails, this article considers a threshold policy on the reservation of agents for the inbound calls. We study a general non-stationary model where calls arrive according to a non-homogeneous Poisson process. The optimization problem consists in maximizing the throughput of emails under a constraint on the waiting time of inbound calls. An efficient adaptive threshold policy is proposed that is easy to implement in the automatic call distributor. This scheduling policy is evaluated through a comparison with the optimal performance measures found in the case of a constant arrival rate and also with other intuitive adaptive threshold policies in the general non-stationary case.