What is the best way to correct a shifting calf implant?

It sounds as though the current implant pocket is too large. Typically the only reliable way to fix this is by creating a completely new pocket just deep to the current one. If this is not possible then a flap of tissue at the top part only may be raised up to contain the implant and prevent “shifting” upwards. Suturing an implant is often not successful because the suture either tears through the tissue or through the implant. Glad to help. 

This is a problem that can occur with calf implants for no one's fault. 
Because of the dynamic nature of the legs, there are a lot of forces that shift the implant and can cause the pocket to widen. There are various options to correct this: 
1- If the implant in subcutaneous (over the fascia), they can be placed under the fascia
2- If they are already under the fascia, the most effective way to correct his is to remove the implant, allow the pocket 6 months to fully seal, then put a new implant. A more simple, but less successful way, is to open the pocket, extend it further down (for the implant to drop down), remove the capsule (partial capsulectomy) on the upper part of the pocket and sutures the tissue together. This should be followed by a strict recovery regim using specific bandage and limited activities to allow the implant to stay low in place while the upper tissue is sealing close. 

When a calf implant pocket is displaced superiorly (towards the incision) this suggests that the lower end of the implant pocket has not been made low enough. (thus the implant slides superiorly) It is also possible that the implant is actually not under the calf muscle fascia and is in the subcutaneous plane. Without seeing pictures of the actual problem (calf muscles flexed and extended) it is not possible to make an actual diagnosis. But clearly this is an implant pocket issue. But I would have no confidence that trying to suture an implant into place is going to work. Soft solid silicone implants will not hold sutures and it will just tear through the implant.